A great deal of interest is presently being given to the problem of limiting the mass of particulate matter emitted with the exhaust gases from diesel and other internal combustion engines. In the case of diesel engines, work is under way to develop practical and efficient devices and methods for reducing emissions of the largely carbonaceous particulates present in the diesel engine exhaust gases.
It is recognized that one method for accomplishing this purpose is to provide suitable particulate traps such as filters in the engine or vehicle exhaust systems. Two types of filters that have been considered for this application are depth-type filters and wall-flow monolithic filters. The depth-type filters can relatively easily be regenerated in use by incineration. However, high-efficiency depth-type filters have a high pressure drop. Wall-flow monoliths, on the other hand, can provide high efficiency at low pressure drops. U.S. patent applications Ser. Nos. 99,933 now abandoned and 99,935 now U.S. Pat. No. 4,276,071, both filed Dec. 3, 1979 and assigned to the assignee of the present invention, disclose certain forms of ceramic wall-flow monoliths capable of highly efficient filtration of engine exhaust particulates, including the largely carbonaceous particulates present in diesel engine exhaust. Experience has shown, however, that the incineration cleaning of wall-flow monolithic filters is difficult. Methods used for ignition and combustion of deposits during operation of depth filters have not given satisfactory incineration in monoliths. Therefore, new approaches are required to design a filter with the combined properties of low pressure drop, high efficiency and acceptable regenerability.